The present invention relates to a system for controlling a brushless motor in which a current intermittently flows in each of the driving coils in response to the conductive condition of the corresponding contactless switching element such as a semiconductor switching element.
In general, a brushless motor has a plurality of driving coils and contactless switching elements and continues to rotate by an intermittent current flowing in each of the driving coils in response to the operation of the corresponding switching element driven by a rotor position signal.
As a system for controlling the speed of such a motor, there is well known a system having a chopping transistor, an inductor and a diode for current-feedback (fly-wheel diode). In such a system, the voltage across the power source is applied at a certain period to the driving coils through the switching elements in response to the conductive condition of the chopping transistor.
The rotating speed of the motor is controlled by changing the ratio of the conducting period of the chopping transistor to the nonconducting period thereof so as to change the average voltage applied to the driving coils of the motor.
However, such a prior art control system requires the chopping transistor for high power, an inductor and a diode for current feedback. Therefore, this system is complicated and is expensive.